Nuclear measuring system



July 26, 1960 S. A. SCHERBATSKOY NUCLEAR MEASURING SYSTEM Filed May 2, 1955 RATE M EASURI NG 4 Sheets-Sheet 1 4 sheets-sheet 2 Ihlll' MULTICHANNEL PULSE HEIGHT cOlNclOENOE NETWORKS I g 3oz S. A. SCHERBATSKOY NUCLEAR MEASURING SYSTEM July 26, 1960 Filed May 2. 1955 CONTROLLABLE l 50o l CHANNEL Fig. 9

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P HOTON .f LlNcOMING PHOTON INCOMING July 26, 1960 s. ,1a.scHERBATsKoYl 2,946,888

NUCLEAR MEAsuRING SYSTEM 4 Sheets-Sheet 3 Filed May 2, 1955 INVENTOR.

July 26, 1960 s. A. scHx-:RBATSKOY 2,946,888

NUCLEAR MEASURING SYSTEM Filed May 2, 1955 4 Sheets-Sheet 4 224 ADDER 222 "Ef-5T ./500 5? 50e 25o oolNclDENcE NETWORK UnitedStates Patent O NUCLEAR MEASURING SYSTEM Serge A. Scherbatskoy, 804 vWright Bldg., Tulsa, Okla.

Filed May 2, l1955, Ser. No. 505,086

10 Claims. (Cl. Z50-71.5)

This invention is concerned with an apparatus for performing measurementsv of radiations' resulting from nuclear transformations within an unknown substance the characteristics of which it is desired to determine. The nuclear transformations can be caused by an external agent such as a neutron source placed adjacent to the substance and in the neighborhood of a suitable detecting instrument.

Various specific objects of my invention and the details of its operation will be specifically described in connection with the accompanying drawingsin which:

Fig. 1 shows schematically a device for performing measurements n'a bore hole in the earth as practiced in the prior art.

Fig. 2 -shows schematically the application of Y'my measuringk system for operation in a bore hole.

Fig. 3a shows a threshold network for transmitting impulses below a determined thresholdV value.

Fig. 3b shows a threshold network for transmitting impulses above'a determined threshold value.

Fig. 4 shows the application of the principles of my invention for determining the thickness of a plate.

Fig. 5 shows the application of the principles of my invention for determining the level of a liquid in a tank.

Fig. 6 shows the application of the principle of `my invention for determining the density of uid within a pipe line.

Fig. 7 shows the assembly of crystals to beused in connection with Fig. 2 to produce pulses having magn itudes proportional to the energy of incoming photons.

Fig. 8 shows the photon tracks in a crystal assembly shown in Fig. 7.

Fig. 9 shows a modification of the arrangement shown in Fig. 7.

Pigs. 10a and 10b show diagrammatically a controllable adding network that is part of the arrangement of Fig. 7. t t

Fig. 1l shows diagrammatically a controllable channel that is a part of the arrangement of Fig. 9.

Referring now particularly to Fig. 1, numeral 10 designates an exploring housing which is adapted to be loweredtby means of a cable to various depths in the bore hole 11 in order to perform measurements of the formations adjacent said hole. The exploring housing contains essentially three elements: (l) Ya source of radiation 12 such as a source of neutrons or gamma scattered by the adjoining formation'and to be insensi- The pur-,

. 2,946,888 Patented July 26, 1960 ICC tive to any direct radiations from the source 12. Therefore because of the shield 15 the output of the detector 14 can Abe used as an index of the properties of the adjoining formation. It is apparent that if the shield were not present, the direct radiations from the source to the detector would be intense, and any useful eifect due to the radiations scattered and reflected by the adjoining formation would be completely lost and masked by the elfect of the source.

In the prior art the presence of the shield introduced a certain inexibility in the design of the subsurface exploring instrument; namely, the source 12 could not be placed arbitrarily with respect to the detector 14,

' since there had to be a certain minimum distance d between the detector and the source that was sufficient for the insertion of the shield 15.

tion by eliminating entirely the shield and utilizing as detector a proportional counter, that is adapted to produce impulses that are proportional to the energy of the intercepted radiation particles or quanta. known that the radiation that follows the direct path from the source to the detecto-r is different in energy (and in some instances inv character) from-the radiation that is scattered by or induced in the formation. Accordingly, I provide across the output terminals of the proportional counter a pulse height analyzer that selectively transmit-s only the impulses within an energy range corresponding to radiation scattered or induced in the formation and selectively attenuates the impulses within an energy range corresponding to the radiation that is directlytransmitted from the source to the detecten It, is

thus apparent that such a pulse height analyzer makes a For the purpose of exploring the formations along the' bore hole there is provided in accordance with the present'invention exploration apparatus comprising a housing 22 which is lowered into the bore hole 21 by means of a cable 24. The cable 24 has a length somewhat in excess of the depth of the bore holerto be explored and is normally wound on a drum to lower the exploring apparatus into the bore hole 20 and may be rewound upon the drum to raise the exploring apparatus.

In order to determine the depth of the exploratory ap-.

paratus withiny the bore hole, measuring wheel 23 is provided which measuresvthe depth in a conventional II'laIlIleI'.

The housing of the exploratory apparatus is vdivided into three sections designated by numerals 31',A 32, and`4 33, respectively. Y In the section 31 there is provided a solid support 35 on which is disposed a suitable source..

of radiation 37 to be described hereafter.

' photons.

' The section 32 comprises a scintillation.counterl 39, consisting 'of a crystal 40 anda photomultiplier 4L The crystal 40 may be of anthracene, sodium iodide,

or sodium tungstate or of any other substance adapted to produce light as a result of interaction with incident contributes a portion of its energy to Compton scattering at each scattering point. Itis desirable for the incident nphotonto. be completely absorbed; by th/Qrystali It is well As it is well known,- an incident photon inter-` acting with the crystal undergoes multiple scattering and` since only then will the amount of light produced in the crystal be proportional-to Vthe photons'energy. The complete absorption of the gamma ray Within the crystal can be accomplished by selecting a sufiiciently Ylarge crystal. The light produced in the crystal vas -a result of interaction. -with the gamma vray vsubsequentlyirnpinges upon a photomultiplier 4,1 to produce a current impulse proportional to the energy offthe intercepted gamma yray.

The output of thefphotomultiplier A#lis in turn yapplied to Itheinput terminals 42 of -a-thresholdnetwork designated schematically by block 43 and located `int-he c ompartment 33. The -output of Athe VAthreshold -network -is in turn connected to the cable 24. An-ampliiier 45 is connected -to the surface end of the cable 24. 'Ifhe output of the ampli-fier is connected to the ratemeasuring network 46 which vis Yof conventional typegandis adapted to produce across itsoutput terminals 47 a D C. voltage having .a magnitude representing Ithe frequency of occurrence ofthe impulses applied to its input. vThe output of -the rate measuring network is connected -to the recorderl. Y

To illustrate my invention, I shall'present =two examples: ln the first case, 37 shall designate a source of gamma rays such vas C060 and the detector 39-will detect gamma rays scattered by the adjacent formations. 'In the second case, 37 shall designate a neutron source such as radium beryllium mixture and the detector 39 will detect the gamma rays resulting from gthe capture of lneutrons by various elements :that are present in the adjacent formations.

Consider now the irst example based on the use of a source of gamma rays to irradiate the formations and of adetector .of gamma rays to detect and measure the gamma rays scattered by the formations. In lthis embodiment the numeral 37 designates a C060 source which emits gamma rays of energy about 1.2 mev. (More exactly it emits two `monochromatic rays having energies 1.17 mev. and 1.33 mev.) These gamma rays arrive at the detector either asa direct radiation or as a radiation that is scattered :by the adjoining formations. It is apparent that directradiation suifers no degradation of energy and the photons that are directly emitted bythe source 37 and interact with the crystal 40 produce in the output of the photomultiplier 41 impulses having magnitudes corresponding to the energy of 1.2 mev. On the other hand, those gamma rays that irradiate the adjoining formations sufer a degradation of energy since they undergo Compton scattering in which the 4scattered photon has only a portion of the energy of the incoming photon. Therefore, the electrons scattered '.by the formations interact with the crystal .and since these electrons have an enengy bellow v1.2 mev. the corresponding impulses Yproduced by the multiplier are smaller than the impulses due to the photons following the direct path from the source 37 to the crystal 40. The threshold network 43 connected to the output of photomultiplier V41 selectively transmits only -those impulses that have magnitudes smaller than M. where 'M designates the magnitude of impulses corresponding to an energy `-of 41.2

mev.

The impulses having magnitudes smaller than M are transmitted Vby means of the cable 24 to the y top Yof the drill hole, amplied in the amplifier 4 5 and applied vto the rate measuring network 46. 'The voltage output of 45 is in turn applied to the recorder 31. Thus the variation of this voltage with the depth will 'be Shown 0n the recorder 31 While the depth of the subsurface instrument in the hole will be indicated by means vof the cable measuring device l,23.

In Athe second embodiment of my invention I use ,a source of neutrons to irradiate the formations and a detector responsive to the gamma rays of capture emitted by said formations. Numeral ,3'7 designates `a radium beryllium preparation which may be enclosed in a container made of a suitable material such as glass` As it is well known, the radium beryllium mixture is not a pure source of neutrons since it emits a heterogeneous radiation comprising neutrons and gamma rays. The major portion of the emitted gamma rays are due to ra-v dium in equilibrium with its products and their energies are below the value of 2.5 mev. v

A portion of the beam of neutrons and gamma rays emitted by the source 37 travels :directly from the source to the crystal 40 and the remaining portioninteracts with the adjoining formation. The crystal 4i) 'is composed of heavy elements such as calcium tungstate tand, therefore, Vit does not respond to fast neutrons thatarrive directly from the source. It responds, however, to gamma rays and since these gamma `rays have energies below 2.5 mev., the current pulses in the output of the photomultiplier having magnitude below a certain value N, said Value N corresponding to energies of 2.5 mev.

The lgamma radiations emitted by the source 37 undergo numerous collisions in the adjoining 'formations and are partly scattered towards the detector. These scattered gamma rays have energies that are lower than the primary gamma `rays emitted by the source 37. Conse-V quently the current impulses in the output of thephotomultiplier -41 that are due tothe scattered Ygamma rays are smaller than the above referred to value N.

The high energy neutrons emitted by the source .37 are slowed downto thermal velocities and then diffuse a distance which is determined by the abundance and capture cross sectionsof the elements kpresent and eventually becomes absorbed by various elements. Upon Vthe absorption of a thermal neutron, each elementremits a gamma ray called gamma ray .of ,captureandhaving 3an energy characterizing a given element. For'instance, an atom of hydrogen by .capturing a neutron emits agamrna ray of energy 2.2 mev., an atom ofnitrogen by'capturing a vneutron emits a gamma ray of energy 10.78 mev., an atom of aluminum by capturing aneutron emits a gamma ray of .energy k8 mev.

It should be noted ,that hydrogen .emits algamma ray ofcapture having energy smaller Vthan 2.5 vmev., and that all other elements -ernit ,gamma .rays of .capture'liaving energies higher than 2.5 mev. Thus the current impulses having magnitudes smaller than N `correspond `.toneutron capture by hydrogen and those 'having magnitudes larger than N correspond to capture by `all remaining elements.

The .threshold network 43 connected to :the output of the photomultiplier 411. is adapted .to transmit selectively only those impulses that exceed the value N and selectively attenuate the impulses smaller than N.. The impulses exceeding .the value .N are :transmitted rby means of the cable 24 to the top of the drill hole, amplified in the amplifier 45 and applied .to the rate measuring network 46. The voltage output of 46 is in turn applied .to the recorder 31.

The threshold network 43 is used ltopperforrn the -following functions: (l) to selectively attenuateand eliminate gamma rays that arrive directly .from the source to the detector, i.e. to eliminate the shield customarily used to shield the source from the detector; and (2A) to selectively attenuate 4 and eliminate Vfrom the recording the gamma frays .of capture Adue to hydrogen so as to produce a log representing the relative ,abundanceof .heavy elements; vand (,3() to selectively attenuate and :to eliminate from `the recording Ygamma rays that areemittedby .the naturally `radioactive elements present in the formation since :thesegamma rays have energies `below 2.5 mev. It

iS thus .apparent that I have .provided in my second em-v bodiment a Jog representing the .relative abundance of .all elements heavier than hydrogen that are `,present .in the formations surrounding the bore hole. l

Figs. `3a and 3b `show the diagrams of the threshold network 43 to be used respectively in connection with the two embodiments of my invention. The threshold shown 'in Fig. 3a is adapted to transmit only 4those irnpulses 'that correspond to 'the gamma rays of energies less 2,946,8ssf

than 1.2 mev. and is to be used in conjunction with a Y source 37 of gamma rays such as Co60 emitting gamma rays of 1.2 mev. As shown in Fig. 3a one of the input terminals 42 is grounded and the other input terminal is connected to the primary windings 91 and 92 of a transformer 93, said windings being arranged in opposition. The ungrounded input terminal is connected to the winding 92 through a resistor 94 and to the winding 91 through a channel comprising a rectifier 95 in series with a battery 96. The secondaryV winding 97 of the ytransformer 93 is connected to the output terminals 44.

The voltage of the battery 96 is arranged to oppose the voltage across the input terminals 42. Thus whenever a pulse exceeding a certain threshold value (corresponding to the energy of 1.2 mev.) appears across the terminals 42 we obtain a current owing through the rectifier 9S and the winding 9'1 to the ground terminals in the direction of the arrow A and another current simultaneously flow through the resistor 94 and the Winding 92. to the ground inthe direction of the arrow B. These two currents produce iiuxes in the opposite directions in the primary of the transformer 93 and therefore no voltage appears across the output terminals 44.

On the other hand, whenever the pulse applied to the input terminals 42 is below said threshold value, the opposing voltage of the battery 96 Vexceeds the voltage provided by the pulse and therefore no current can flow through the zrectifier 95 in the direction of the arrow A and the winding 91 is de-energized. However, a current flows through the resistor 94 and the winding 92 in the direction of the arrow B and this current induces a voltage across 4the output terminals 44 of the transformer 93. `It is thus apparent that the network shown in Fig. 3a transmits only those impulses that are below a certain threshold value, said threshold value being determined by the battery 96.

The threshold network shown in Fig. 3b is adapted to transmit only those impulsesl that correspond to gamma raysv of energies above 2.5 mev. and is to be used in conjunction with a source 37 consisting of a neutronberyllium mixture. As shown in Fig. 3b the threshold channel comprises a battery `80 in series with a rectier 81 and resistors 82 and 83 interposed between the input terminals 42. The output terminals 44 of the threshold network are applied to the resistor'` 82. The voltage of the battery 80 is so arranged as to oppose the voltage across the input 4terminals 42. If the voltage across the input terminals 42 is smaller than the opposing voltage of the battery there is no current in the circuit becausel of the'unidirectional action of the rectifier 81. However, if the voltage impulses across the input terminals 42 exceed the voltage of the opposing battery 80 we obtain a current that is transmitted through the rectifier 81 and causes a corresponding voltage across the output `terminals 44 of the resistor 82. The voltage of the battery 80 determines the threshold and thus only Vthe voltage impulses exceeding the threshold appear across the output terminals 44. On the other hand,v the voltage impulses at the input terminals 42 that are below the threshold are not transmitted because of the unidirectional conductivity ofthe rectifier 81. f

Figs. 4, 5, and 6 show some applications of my inven-A tion to non-destructive testing. The testing instrument shown in these figures iscontained in a box or casing 100. Within rthe casing 100 a source 101 of gamma rays such as C060 is placed adjacently to a scintillation counter comprising a crystal 102 which scintillates when intercepted by gamma rays and actuates the photomultiplier 103. The-crystal is of relatively large size so as to ab# sorb completely the intercepted gamma rays and-consequently, we obtain across the output terminalsof the photomultiplier 103 a succession of impulses having magnitudes representing energies of the intercepted photons.

Some of these photons `arrive directly from the sourceV 10.1 and havethe energyv of 1.2 tuev., The remaining 6 photons that intercept the crystal`102 are scattered by the surrounding medium and have energies less than 1.2 mev. The output impulses from the photomultiplier 103 are transmitted through a threshold network 104. The threshold network is of the type shown in Fig. 3a,l i.e. it transmits only those impulses that correspond to energies less than 1.2 mev.r Thus the impulses transmitted by the network 104 correspond only to those photons that vhave been scattered by the surrounding medium. These impulses are in turn transmittedto a rate measuring net work 105. The output of 105 is in turn applied to the indicator 106. l y

Fig. 4 shows the application of my invention to the determination of the thickness l of a plate'110 from one side only without any necessity for obtaining access to the other side of the plate. Such a measurement can be used either inside or outside of steel tubing or lother similar forms and can be rapid and easily performed. The principle of this measurement is based on the well known physical principlevthat radiation passing through matter will be scattered and the amount of radiation scattered will increase with the amount of matter traversed. Thus the gamma rays from the source 101 are scattered by the plate 110 and the amount of scattered radiation is proportional to the material traversed, i.e. to the thickness of the sheet 110. VThus the indication of the meter 105 represents thev thickness of the platev 110 and if the indication varies it will indicate corresponding variations in the thickness.

Fig. 5 representsythe application of my method to the determination of the level of liquid in oil tanks. Let 111 designate an oil tank which is filled to the level CD with oil, said level being at a height H. It is desired to produce a signal whenever the oil reaches the level ABv at a height H. This is accomplished by means of the instrument which is fastened to the outside wall of the tank at the level H as shown in Fig. 5. It is apparent that the meter 106 indicates radiations that are scattered noty only by the wall of the tank adjacent to the instru-v ment, but also rays that are scattered by the oil within the tank. If the liquid level is at a height H which is below the critical height H the scattering of the gamma rays takes place in the wall of the tank and in the air within the tank. Because of the low density of air the scattering is relatively small and the meter 106 indicates a low reading. However, when the liquid level reaches the height H the amount of the scattered radiation as' dication of the meter 106 `can serve to measure the den? sity of the fluid within the pipe.

Fig. 7 shows an improved arrangement (that can be used instead of the arrangement 39 of Fig. 2) for de# tecting gamma rays and for producing current impulses y having magnitudes proportional to the energy of the l:gamma rays.

the central crystal and is completely surrounded by the remaining crystals designated as peripheral crystals. The

crystals may be sodium iodide or .anthracene or of any other substance that is adapted to 'emit light under the" eiect of ionizing radiation. In Fig. 7 the central crystal 200 is almost completely surrounded by theperipheralA to the photomultiplier tube 212. Similarly, the crystals' 201Y and 202 are respectively surrounded by refiectors 213,. 214 `which. .direct light produced in said crystals The detecting element in Fig. 7 consists of a plurality of crystals of which one is designated asl The to an adderj 222 by -means -of leads T223 -and .224, said adder being -actuated by thencoincidence network 219 through vleads 225. Theadder2221is adapted togproduce across its output leads 250 an impulse which lis equal-'to the sum of Yimpulses derived .from Ithe photomultipliers 212, 216. 'Normally the adder .222 is inoperative .and becomes actuated :only when -the impulses Vderived -from the photomultiplier 212, .216 4occur in coincidence.

Ther-output .terminals of the photomultipliers .212 .and 217 are applied to acoincidence network 227 by means of 4leads 228 :and 229, respectively, and `are .also applied to an adder 230 by means of leads 231 and 232, .said adder :being vactuated by the coincidence network 227.

Similarly, the voutputeterminalsof all three photomultipliers 212, 21-6, and 217 are applied to .a Ytriple coincidence network L234 by means of leads 2357 .236, and 237, respectively, and are also applied .to an adder 238 by -means of leads 239,240, and v24,1, .said adder being actuated by a coincidence network 234 through leads 242. Consider `now an incoming photonthat interacts with the central crystal 200. Such an interaction lmay consist of a photoelectric effect, a ICompton scattering, .or apair formation. Assume that the incomingphoton has energy in the range between 0.5 mev. Vand 2.5 mev. this energy range, Vthe pair formation is 4negligible and therefore need not be considered. The lphotoelectric effect can also be lneglected particularly if the `crystal comprises elements of'low atomic .number (such :as aiu-V thracene). Consequently, the only phenomenon `.to .be accounted for -is the Compton scattering. The "Compton scattering is essentially a collision of `the incoming photon with one ofthe orbital electrons., Vthatis assumed to be'free. As shownin Fig. 18, Mthe first collision '.takes place at the point Aand the path .of the inco'rning photon arriving at the point A 'is indicated by a dashefd line..

atoms of the crystal. This .energy is .converted Sinto a light pulse 'and theV magnitude 'of :this .pulse is propor-V tionalto the 'en'ergyiof `the recoil electron. iConsequently, the llight .pulse produc-ed in the crystal as `fa l"result .of the iirst Compton scattering is vvnot equal to the -total energy tof the incoming photon, but only .-to thcwportion of `this energy "that has :been contributedV tothe ejected electron.

.As stated above, as a resultof 'the first collision, ethe incoming vphoton contributes to the vejected electron a.

portion 'of its energy and the remainderof the energy of .the .incoming photon :is :carried away lby the=scattered photon. .Assume wnow that the vsecond vscattering voccurs within the crystal ".200 .at `a point LB, 4i.e. fthe .scattered photon .collides with another-electron Within the `crystal 20.0. The :same `process .is repeated, ,i;e. .a `zportion :of the :scattered 'photon is applied to the vejected electron and `.the remainder =of the energy is `carried 'by fthe acattered photon. The electron .ejected at'the point'B l.produces .a flight il'ash the `intensity of which is :proportional to `the energy .of the electron 'ejected :at rB.

The photon :undergoes successive j'collisions'.at points C, B, etc. iA'tt each of :these points :it releases anelec- Inv 8. tron .that :carries o a portion o'f its energy. The paths of the electrons ejected during rrthese multiple :col-

lisions -are designated -by `solid :lines andthe paths :of

the -scattered rphotons are designated vby .dashed lines. After such multiple collisions fthe energy :of .the photon becomes .considerably 'degraded .and `for all practical purposes we 'may .assume the 4energy -:of v.the original :photon vhas been used Aup .entirely to eject :electrons at points zA, B, 1C, etc., each :of .said electrons producing .a light impulse the intensity tof which *is proportional to :its energy. .All these light impulses `toccur simultaneously .and `are transmitted `through the .light "pipe 21-1to'the Iphotoserrsitive surface of :the .photomultiplier 212 `(as 'shown in iFirg. 7e). Thus the :photomultiplier 2'12 receives va total light impulse that is aproporti'onal to .the :total energy of theV incoming photon, and produces across its output terminals -a currentimpnlse having 'magnitude that is proportional tothe energy of said incoming photon.

It is `thus apparentthat the crystal vv200'in conjunction with :the `photomultiplier 212 :produces ipulses proportional .to the energy of incoming photons. In order to establish such a proportionality it is Anecessary that the energy of the lincoming photon be completely absorbed within :the crystal. This .can `be accomplished by taking 'a relatively large crystal so 'that alltthe points such as A, B, C, etc. at which electrons fare .released during the multiple collisions, are contained within l.the crystal. On the other hand, however, by vincreasing the size ofthe :crystal we decrease its transparency and this in turn impairs the proportionality in the detector output.

Furthermore, even if we chose a very large crystal we will never have the assurance that 'each :photon will lose .all vits Venergy withinthe crystal. 'A `relatively common occurrence is illustrated in Fig. -S in ywhich .the incoming photon interacts initially at a point M in the crystal 200 and -ejects an electron A'to which lit imparts a portion fof its energy. SHowever, the remainder vvof the energy is carried'oi by the scattered :photon which escapes Yfrom the central icrystal 200 and loses :its energy in multiple collisions 2at points 'N, C), .P, R within the .adjoining crystal 201. Consequently, fwe obtain two simultaneouslight pulses. I'One pulse occurs Vin 'the crystal 200 in the neighborhood of the pointM and actuates the photomultiplier '212. 'The current .impulse across vthe Voutput terminals of 'the photomultiplier 212 represents therefore the Yportion of the photon fenergy that has ybeen communicated to the Compton electron ejected Aat the `point 4The `other pulse -occurs vin -th'e crystal `201. This other'pulse consists fof 'a superposition of pulses occurring 'simultaneously in the vicinity fof points N, if), P, R. These pulses factonfth'e photoniulti` plier '2116 "and produce KNa current 4impulse across "its output terminals, said current V'impulse representing the undergoes -its `iirst collision 'at `the point T :in vthe 'cry's-{ tal 200 v"and ejects a Compton electron which is only partly 'absorbed in'theicrystal 200. This electronleaves the crystal "200 7and dissipates the rest of its ienergy Ein the crystal 201. VeIh-ejscattered photon `emitted at `the point 4T undergoes the fsecond `and 'third scattering at the Vpoints V and 'W and consequently it Aclissipates 'its energy in 'the 'crystal '201. 'Thus the -s-umof impulsesacross I'the loutput terminals o'f "the photomultiplier 2112 and 2161rcpresents the total energy of `theincorning fpho- J'Oonsider :now again Vthe iagramcf Fig. 7. The in- -rasa-rases :coming photons are intercepted by one or more crystals thus releasing Compton electrons. The Compton electrons produce lightV xllshes thus causing current impulses Vto appear in the output of one or more photomultipliers.

only itsirst collision in the crystal 200 and the remaining collisions take place in the crystal 201, then two impulses appear simultaneously across the outputs of Ithe photo- 'multipliers 212 and 216. These impulses are applied to the coincidence network 219 and tothe adder 222. The adder is normally inoperative fand becomes operative only if actuated by the current from the output-terminal of the coincidence'network 219. Since the two impulses arrive in coincidence 'theyactuate the coincidence network 219 which in turn actuates the adder 222 and consequently we obtain across the output'terminals 250l of the adder the current representing the Asumof these two impulses.r AThus the curren-t across 'the outputterminals 250 represents the energy ofjthe incoming photon.

It is thus apparent that ,whenever an incoming photon is completely absorbed by the crystal 200 we obtain across the terminals 251l of the photomultiplier 212 a current impulse having magnitude representing the energy of said photon.` Whenever an incoming photon is partly absorbed in the crystal 200 and partly in the crystal 201 we obtain across the terminals 250 of the adder 222 a pulse having magnitude representing the energy of said photon. By means of arguments similar to those used above, it can be shown that whenever an incoming photon is partly absorbed in the crystal 200 and partly in the crystal 202 two current impulses appear in coincidence across the output terminals kof the photomultipliers 212 and 217. These impulses actnate the coincidence network 227 which in -turn actuates the adder 230 and We thus obtain across the output -terminals 253 of the adder a current, the magnitude of which represents the energy of the incoming photon. p l

, In some instances the incoming photon may undergo multiple collisions `in crystals 200', 201, and v202, and therefore we obtain simultaneousrpulses in each of these crystals and the total energy released by these three pulses representsthe energy of the incoming photon. Undervthese conditions, the'. photomultipliers 212, 216 and 217 are simultaneously actuated. vThe'out-puts of these photomultipliers 4are applied to a triple `coincidence network 234 through-.channels235, 236, and 2737, respectively, -said network being provided with an output ,channel 242. Furthermore, the outputs of these photomultipliersV are respectively `applied to the adder 238 through the channel 235 in series with 241, the channel 236 in series with 239, and the channel 237 in series with 240, respectively. Consequently, whenever light Ypulses appear simultaneously in the crystals'200, 201, and 202, the coincidence network 234 produces a pulse across its output terminals 242, and this pulse in turn actuates the adder 238. We thus obtain across the output terminals 257 of the adder 238 a pulse having magnitude equal to the sum of the impulse applied to -its input terminals 239, 240, and 241. It is apparentthat the pulse produced by the 'adder represents the magnitude of the photon which interacted with the crystals 200, 201, and 202.

Each of the leads 250, 251, 253and 257 is applied toV vtheinput terminals 255 of the multichannel pulse analyzer 256, which is adapted to separate the pulses in various groups in accordance with their magnitudes. For a description of a pulse height analyzer see, for instance, an article by C. W. Johnstone, A New Pulse-Analyzer Design, Nucleonics, January 1953, pp.` 36-41 to U.S. Patent 2,642,527 issued to G. G. Kelley.

Consider nowFig. 9 representing a-modied embodiment of a proportional gamma ray counter. The por- -tion of Fig. 9 comprising crystals 200, 201, 202 photomultipliers 212, 216, 217 and associated equipment is identical to the corresponding portion of Fig. 7. Those elements that are the same in Fig. 7 and Fig.'9 have been designated in both :figures by the same numerals. Referring now more particularly to Fig. 9 the outputs of the photomul-tipliers 212, 216 are applied to a coincidence network 300, said network havingits output'terminals connected by means of lead 301 -to a controllable channel 3012.. Similarly,l the outputs of photomultipliers 212, 217 are applied to a coincidence network 303, said network having its output terminals connected through leads 304 to said ,controllable channel 302. v

The controllable channel 302 has its input terminals connected to the output of the photom-ultiplier 212 and has its output terminal connected to a multichannel pulse height analyzer 310. Under ordinary operating conditions the channel 302 is'operative, i.e. a voltage applied .to the input terminals 311 is transmitted to the output terminals 312. However, the channel is not operative .whenever either of the coincidence networks 300, 303 is gamma ray, and whenever such a pulse appears there are no coincident .pulses across the outputs of either of the photomultipliers 216 or 217. Consequently, neither of the coincidence networks 300 and 303 is energized and thus the pulse from the output of the photomultiplier 212 1s transmitted through the channel 302 to the pulse height ,analyzer `310.

`Consider now the gamma rays that are onlypartly absorbed within-,the crystal 200. An example of such an incident `gamma ray is shown in Fig. 8 in which the gamma ray undergoes only one scattering at the point M in the', crystal 200 since after the first scattering it escapesk from the crystal 2,00 and undergoes all successive collisions at the points N, O, P, R in the crystal 201. Such a Agamma ray releases only a portion of its` energy within the crystal 200and therefore the resulting, pulse appearing inthe output of the photomultiplier 212 is not indicative of the energy of the incident gamma ray. The purpose of this arrangement is therefore to eliminate from recording any incident gammaray that is only partly absorbed in the crystal 200. Since the crystal 200 is entirely surrounded by the crystals 201 and 202 any photon which is only-partly absorbed in the crystal 200 loses the rest of its energy in one of the adjoining crystals 201 or `202. This is illustrated in Fig. 8 in which the incoming photon following the trajectory M, N, O, P, R is partly absorbed-inthe crystal 200 and partly in 201. It is apparent that in such case two currentimpulses appear in coincidence across the output terminals of the photomultipliers 212 and 216. Consequently, the coincidence Vnetwork 300. is actuated` and a pulse appears across the leads 301. This pulse is applied to the controllablechannel-302 and interrupts the connection between the leads 311 and 312. kThus no limpulse is transmitted from the photomultiplier 212 to the multiple pulse height analyzer 310.

Similarly, whenever an incoming photon is only partly absorbed in the crystal 200 and loses the remainder of its energy inthe crystal 202, we obtain coincident light pulses inthe crystals `200 and 202 which in turn 'energize the photomultipliers 212 and 217 and the, coincidenceV network 303;V The coincidence network produces a pulse across its output leads`304 which in'turn is applied'to the V"aparece 1l 1 -controllable ehnnelr302 and interrupts t-he'con'nec'tion be- Ytweeny the leads 311 and 312. Thus no impulse is transvmitted from the photomultiplier 212 to the multiple pulse #height analyzer 310.

'It isf'thus apparentY that I- have provided a scintillation counter that responds to those photons that are completely vabsorbed'within the crystal 200 and does not'respond to those-photons that arefonly'partly fabsorbed within the crystal' 200.

It isiapparent that I can apply vthe principlesl ofmyin- -ventio'nfto a neutron Vcounter by utilizingin the arrangefments of Fig. 7 and Fig. 9 a crystal made of elements'having a low atomic number such as anthracene. As is well knownya 'neutron interacting with a crystal'under-goes a multiple collision somewhat similar to the one illustrated fin'1Fig.'8. Therefore 'it can vbe entirely abs'orbedin the crystal 200 orfonlyY partlyabsorbed in the crystal 200, `thel remainder.v of .the neutron -energy being labsorbed in one 'of the adjoining lcrystals 201 or 202. Thus the diagramofy Fig. 7 and` Fig.` 9' can be applied to neutron deltec'tionas `well as to. gamma rayl detection. vIn considering the interaction of neutrons in matter, We should keep Yin. mindthatfthe multiple collisions such as illustrated in'Fi-g.=`8'are the collisions of a neutron With lthe photon, vrand'as a result of each collision the photon acquires kinetic-energy that is `dissipated=in "ionizationvv and excitation'and. produces ail-ightI impulse. Thus I have provided -a-*scintilla'tioncounter-that is adapted to produce current Zimpulses representing theenergi'es of/incoming neutrons.

`Consider now Fig. 10a showing diagrammatically an adder such -as the one designated by the num`eral-222 in Fig. -7,"said adderbeing comprised in Fig. 10a`5Withinl a vdottedrectangle. The purpose of the adder `vis to produce across the output terminalsf250` a voltage representing the sum of the input voltages applied across the leads 223 and 224. "Furthermore,'the adder should befeiec- -tive'only Whenever a voltage appears across the control leads 225. The voltages applied acrossthe'leads'223, 224 are applied to the transformers 500 and'501. The secondary windings of thesetransformersare connected Ain series V'and therefore the voltage across the leads 502 is `equal-tothe"sum "of -the`vo1ta`gesacross the input'terminals I223 and 224. The voltage across the leads 502 Ais in' turn applied to theprim'ary winding of a'tran'sformer 503, the secondary windingof said transformer having its terminals'connected to the grids of triodes 504 and 505. -Thetriodes have their platesv connected to the primary -windngfofia transformer 506,`said transformer having its secondary winding connected tothe output terminals 250. :We have thus -arpush pullamplier which under normal 'operating conditionslis-biased to'cut olf byl means ofv a Vbattery Y507. The control voltage'across the terminals -225 -is arranged to opposethe biasing battery 507. Contsequentlynosignal -is-transmitted fromthe channel S02 to the output channel 250 because of the biasing effect of the battery '507. However, whenever-.a voltage appears'acro'ss `the"control terminals 22S the push fpull arrangement becornes etfectiveto transmit the signal from the channel 502to the ot'put terminals 250 andfwe obtain thusaro'ss the outputA terminals 250 a 'voltage representing the'sum of 'voltagesapplied'acro'ssvthe output channels 224'a11d 223.

Fig. v10b 'represents schematcally'the*adder 'suchtas the `one"`designated"as A238 'in Fig. 7. The operationof the adder`238ifis 'identicalfto the adder 222 'and it isself-explanaforysince"theelemenfs ythat 4are the same 'n` both Figs. 10aV and 10b. The main difference between Figs. 10a Aand 1oz; is'thatin Fig. robar is desiredfwadd the impulses attire-three input terminals 239,249, andl241 `whereas in Fig. l10a it is -desired=toadd tl impulses across the twoinput terminals 224'and-222. InFig 10a the voltage representing the sum of the 'threetiuput 4impulses appears across -the-output "terminals n257`of ='the adderf' and Zis foperativel fonlyfwheneverx la control" voltage-:appears'fat Y312, andcontrol'terminals`301'5and304. Under normal operating conditionsl a voltage applied vto vthe-inputfteri `m'inals-311-ap1` ears across the output terminals-312 and vvthefc'o'nnection' between theinput channel and Vthe soutput lchannel--is'interrupted only if a'voltageappearsatieither ofthe control channels 301,- 304. Asshownfin thegure, the input terminals' 311 are connected to the pri'niaryfwind- -ingof the transformer 510and the-"secondary winding-of -the transformer 510 is connected to thegrids-of the triodes A511 and 512 arranged -in push pull. n

Under Y normal Voperating conditions, l'in' the absence Vof voltage at either of the control terminals l1*01,'$04,'jthe voltage applied to the inputI terminals 311' is transmitted through the transformer' 510? and the pushV pull'circuit to the'output terminals 312. 1 However, whenever a control voltage appears at either ofthe term-inals`301, 304, 'itbiases theanipliierssll, 512I tothe cut' off, and thereforethe -push pull *circuit ``disconi1ects-the input leads 13 11 from the output leads- 312.

"I claim: y I

Vl. IiRadiation-detecting apparatus comprising 'two'sc'intillating' elements adapted yto interact with incident gamma 'rays'fandto 4produce light flashes esponsivelyto such interactio'nsgthe intensity ofsaid respective `iflashes'epre senting 1' the amounts -of energy absorbed` inf said' element from Csaid 'gamma rays in `tlie`- course v'offfsaid respective interactions; a photor'nultiplier -for`-l each V`of Isaid scintillatling v"elements," each of said`"photornultipliers-''being individually optically c'opledtolne-of said`sci`i1tillatingele `ments and being; adapted to produce current-'impulses' having magnitudes respectively represehtingth'e intensi'tyi of the light Aflashes'V occurring inthef scintillatingeelementto 'which it is coupled, and`'meansifed1 bythe impulsesr'from Asaid photornul'ti'pliers operativeselectivelytoadd"'thedrnpulses from said V"respectiv`e` photmultipliers'tliat occur in time coincidence and yto produce therefromnewjimpulses' having magnitudes proportinaltd the s'ir`1`s`-'of'the lrngnitu'desof said selectively-"added impulses. l52.' Radiation-detecting apparatus' comprising g 'two scintillating elementsada'pted to'interact'with5 incident lgar'rirna raysand vto 'produce lightflash'es "fre'spo'risively" A-tosu'ch interactions,'the intensity ofi-'saidlresi `ct`ive flashes representingthe 'amounts of 'energy absorbed in'l'saidelerncnt 'from *said* g'am'rna 1 rays "in '-tlie '^`cou'"rse "of 1Vsaid respective interactions, a photo'rnltiplier for eachbf `said scintillating* elements, feach "of said photomultipli'ersf. ,being individually opticallyV coupled to "one' ofsaid scintillating' elements "and being'adapted topro'dcef urreutfirpulses havingrriagnitudesirespectively repres 'f "g the intensity of the light 'llaslies occurring in thescintillating 'element to'lwhichit isf'c'upled, a coincidence networkc'oupled 'to the outputs' of said" photoniultipli'ers *and* 'peative'to sense the `si'nnilt'alreous occurrence "of limpulsesinfbth 'of1 s d"photornultipliersaiidadder- Y, said coincidence netwok'andfedby saldlp'hotornultiplier impulses :operative: to .add the'rnagiiitudesof output iin- -pulses "fronrsaid respectivephotorn lt'ipli'esoccurigih 'tir'n'emcoir'icitleneead ftoproidlce'th refrom 'new impules ftin'ag'n'itudes 'propontonalto thersums'ifithef magnitudes of said timeicoincdent'jimpulses.

i Radiatiomdetecting lapparatus comprising' a first scintillating element, a second scintillatingelernent largly surrounding said first element, 'both of said yelementsbeing adapted to interact ,with incident gamma-rayphotons and -toprodu'ce #light''ashes Yrespensisely- =theretof`the"i11 tensityfff said respective llight rvflashes inl ea'chof `'said ele from said photons pursuant to said respective interactions,

-.said first and second elements being separated by a layer' Aof material opaque to light flashes, photomultiplier means coupled individually to each of said elements, each ofsaid photomultiplier means being adapted to produce currentVY impulses havingmagnitudes proportional to the intensity of light flashes in its associated scintillating element,A

means fed by the outputs of said respective photomultiplier means for selectively addingy impulses occurring simultaneously at the outputs ofboth of said photomultipli- 'ers and producing therefrom new impulses having magnitudes proportional tov the sums Vof the magnitudes of said time-coincident impulses, and means for selectively'transmitting said new impulses and said output impulses fromV the photomultiplier coupled to said rst scintillating element.

Y 5. Radiation-detecting means comprising a iirst radiation-sensitive' element operative to produce a series of energetic physical events responsively to interactions with gamma-ray photons, a second radiation-sensitive element tion-sensitive elements operative to sense the occurrenceV of said energetic physical events and to produce electrical impulses substantially coincident in time with said events andV having magnitudes proportional to the respective values of said physical property, and means fed by said impulses from both of said detector means operative to add the impulses from said detector means that occur in time coincidence and to generate new impulses having magnitudes respectively proportional to the sum of the magnitudes of said added impulses, said new impulses representing the energies of gamma rays interacting iirst in one of said elements, scattering into the other of said elements, and thereupon interacting in said other element.

6. Radiation-detecting means comprising a rst radia- Y tion-sensitive element operative to produce a yseries of energetic physical events responsively to interactions with gamma-ray photons, a second radiation-sensitive element of like properties, said second element being positioned relative to said iirst element to provide a substantial probability that photons scattered from said iirst element will enter and interact with said second element, each individual energeticvphysical event produced in said elernents being respectively proportional in some physical property to the quantity of` energy absorbed by such element in the course of the gamma-ray interaction producing it, individual detector means for each of said radiation-sensitive elements operative to sense the occurrence of said energetic physical events and to produce electrical impulses substantially coincident in time with said events and having magnitudes proportional to the respective values of said physical property, and means fed by said impulses from both of said detector means operative to add the impulses `from said detector means that occur in time coincidence and to generate new im-V pulses having magnitudes respectively proportional to the sum of the magnitudes of said added impulses, said new impulses representing the energies of gamma rays interacting rst in one of said elements, scattering into the other of said elements, and thereupon interacting in said other element, said last-mentioned means beingioperative to suppress all other impulses fed to it by said respective detector means. Y

7. Radiation-detecting means comprising a iirst radiation-sensitive element operative to produce a series of energetic physical events responsively to interactions with 'ments representing the quantity of energy absorbed therein K gamma-ray photons, a second radiation-sensitive element `of like properties, said second element being positioned relative to said rst'velement to provide a substantial prob- `ability'that photons scattered from said rst element will enter and interact with said second element, each individual energeticphysical event producedV insaid elements being respectively proportional in some physical property to the quantity of energy absorbed by such element in the course of the gamma-ray interactionV producing it, individual detector means for each of said radiation-sensitive elements operative to sense the occur rence of said energetic physical events and to produce electrical impulses substantially coincident Vin time with said events andhaving magnitudes proportional to the respective values of said physical property, and time selective means fed by said impulses from both of said `detector means operative to produce and selectively trans- ,mit resultant electric impulses only in response lto reception of electric impulses from both of said detectors in .substantial time coincidence, yeach of said resultant im:

pulses having magnitude proportional to the sum of mag- .nitudes of `said coincidentally occurring impulses.

' 8. Radiation-detecting apparatus comprising .two scintillating elements adapted to interact with incident gamma rays and to produce light flashes responsively to such interactions, theintensity of said respective flashes representing the amounts of energy absorbed in said respective elements from said gamma rays in the course of such interactions, a photomultiplier for each of said scintillating elements, each of said photomultipliers being individually optically coupled to its associated scintillating element and being adapted to produce electric impulses having magnitudes respectively representing the intensity of lthe light dashes occurring in said associated scintillatphotomultipliers.

9. Radiation-detecting apparatus comprising two scintillating elements adapted to interact with incident gamma rays and to produce light ilashes responsively to such interactions, the intensity of said respective ilashes repv resenting the amounts of energy absorbed in said element from said gamma rays in the course of'said respective interactions, a photomultiplier for each of said scintillating elements, each of said photomultipliers being individually optically coupled to one of said scintillating elements and being adapted to produce current impulses having magnitudes respectively representing the intensity of the lightiiashes occurring in the scintillating element fr to which it is coupled, and means fed by the impulses 'from said photomultipliers operative selectively to add the impulses from said respective photomultipliers that occur in time coincidence and to produce therefrom new impulses having magnitudes proportional to the sums of the magnitudes of said selectively added impulses, said last means being also operative to transmit impulses produced by one of said photomultipliers that are not in coincidence with the impulses produced by the other of said photomultipliers and to suppress impulses produced by said other photomultiplier that are not in coincidence with impulses produced by said one photomultiplier.

l0. Radiation-detecting means comprising aiirst radiation-sensitive element operative to produce a series of energetic physical events responsively to interactions with gamma-ray photons, a second radiation-sensitive element of like properties, said second element being positioned relative to said first element to provide a substantial probability that photons scattered lfrom said -irst element will enter and interact with said second element, each individual energetic physical event produced in said'ele- `15 mentsc'beiiig respectively proportional in some physical prope'rty'to vthe quantity of energy absorbed' by lsuch ele- `ment in the course of the gamma-ray interaction producing it,-individual' detector means for each of said radiation'sensitive elements operative to sense the occurrence -of said: energetic 'physical events' and to produce electricalfimpulses substantially 'coincident intime "With said veventsfand vhavingv magnitudes proportional to `the respcctive values of saidiphysicalproperty,Wherebysorne ofthel impulses produced by said respective detector means foccur Vin coincidence and other impulses produced said yrespective detector mei'ins are not in coincidence, `and timeselective'meansfed by said irng'iulses' from Vboth V'of said detector means operative to add' the impulses from -s'aidl detectors that occur in coincidence and to generate new vifi'mplses having magnitudes respectively proportiona1`t0`-the` sum of magnitudes of said added impulses, Said 'new impulses representing the energies of-gamma raysfinteracfing first'in one of 'said elements, scattering nto the Ao'tllerof said elements, and thereupon interacting in" Said` other element, "said time-selective means being also 'operative lto Vtransmit selectively those ;im

pulses that-'are produced by one ofsaid detector means "VRefeenc'es" Cited lin; the lei of this patent VUNITED STATES PATENTS OTHERv REFERENCES Two-Crystal Gamma-Ray Scintillatin Spectrometer, by R.l E. Connally, from The Review of Scientific Instru- 20 ments, -vol. 24,` No. 6, June l953,pp. 4584-459.

yLarge-Volume Liquid sciiiiiilaiors; Their vApplications, vby Harrison et al., from Nucleonics, vol. l2, No. 3, March l954,`pp. 44'47. 

